As far as the UI is concerned, whenever either Price or Quantity changes on that object, the display of Total should refresh accordingly. In other words, Total depends on both Price and Quantity. I wanted to represent this dependency decorating the property with an attribute in the following manner:

Definitely nothing special there. Next, changes to the DynamicDtoDecorator. Starting with a property to keep a dictionary of property dependencies (the key is the name of the property that has dependencies and the value is a list of properties that it depends on):readonly Dictionary<string, IEnumerable<string>> _propertyDependencies = new Dictionary<string, IEnumerable<string>>();We then change the RegisterProperties method so it verifies whether the property its registering has dependencies, and if so, it calls out the RegisterPropertyDependencies method:

The code simply looks through the registered properties that have dependencies and raise the event as appropriate.

This pretty much wraps up all the main pieces for getting rid of direct implementation of INotifiyPropertyChanged in ViewModels by using the C# Dynamic features. I’ll be keeping the latest version of the code in this repository, so feel free to check it out. I’ll be writing new posts for other improvements I’ve added to the decorator and you might find useful. Stay tuned!

Since the decorator variable above has been declared as dynamic, the compiler won’t complain about the DynamicDtoDecorator not having such a thing as a ProductName property. However, during runtime, the code will blow up when it tries to access that property. Here’s what we need to do to the DynamicDtoDecorator class so it knows how to set a value to a dynamic property:

Our DynamicDtoDecorator class inherits from DynamicObject, which provides a TrySetMember method: this method gets called during runtime whenever we try to set a member on the object. Its SetMemberBinder parameter has a Name property, which gives us the name of the member we’re trying to set (in earlier example, that’d be the “ProductName” property). The method also takes in the value we’re trying to set the member to. Finally, the method is supposed to return true or false in order to indicate whether it could successfully set the value or not.

I’ve created a SetProperty method, which is called by the TrySetMember method, just to keep things more organized. This method performs a some simple validation (making sure the given property name is registered within the decorator), sets the value on the property using our PropertyAccessor, and calls RaisePropertyChanged.

The code that allows us to get the value of a property is very similar:

So, whenever we try to get the value of a dynamic property, a TryGetMember method is called. It takes in a GetMemberBinder, which gives us the name of the member we’re trying to access, and it also takes in an out parameter to which we set what value we want returned out of that operation. The method itself needs to return a boolean indicating whether or not the method succeeded.

That’s it! The most basic implementation of our decorator is ready: we can instantiate it passing in a ViewModel or other type of DTO, set it to the DataContext in WPF visual elements, and use DataBinding. The data displays on the UI, and should values in properties change, the UI should be updated to reflect the change.

So far, what we’re calling a decorator is really a proxy, since it doesn’t really add much functionality to the underlying object. However, once I got to this point, I started seeing other things I could use this approach for, and the class really became a decorator. But more on that on some other upcoming posts. We’re not done with this series yet!

On this part of this series, we start looking at the DynamicDtoDecorator class. We begin by seeing how it takes in the object that it decorates and how it figures out what the public instance properties in the decorated object are, and how to access them. We’ll be seeing how we use the PropertyAccessor class created in the previous installment of this series.

The class inherits from DynamicObject so we can get some dynamic support for free. It also implements INotifyPropertyChanged. “But aren’t we trying to get rid of INotifyPropertyChanged?”. Yes, we’re trying to get rid of it in our ViewModels, but we still need it somewhere for WPF binding to work.

Next, we have a private field to store the DTO (Data Transfer Object) that the class decorates. The DTO is passed into the class constructor

The method looks for any public instance property on the DTO type, creates getter and setter delegates, and register them with in the decorator. If you’ve been following my series on this topic, you’ll remember my explanation on a PropertyAccessor class I created in the previous installment, so you can already think how those getter/setter delegates are going to be used, right? Well, alright, I forget things all the time, too, so let’s see what that Register method looks like:

el;In Part 1, I mentioned I wanted to decorate a ViewModel with “notify property changed” behavior, so I could keep my ViewModel as clean as possible. In this part I’ll go over how the properties on the decorated object are going to be accessed.

The important aspect in the code above is that the decorator has to be able to access properties on the decorated object. In order to come up with a generic way to do that, I’ve created a PropertyAccessor class, designed to get/set value on a property.

The PropertyAccessor class uses a Func delegate to do the “get” and an Action delegate to do the “set”, like so:

That’s as simple as the class can get. That’s how I wanted my ViewModels to look like. Notice it doesn’t even inherit from any baseclass, and it doesn’t have any attributes decorating the class (the way I also did at some point when using IL weaving tools such as PostSharp).

WPF databinding is late-bound, so as long as the property exists on the object during runtime, everything works just fine. When it comes to changing data in the UI and having WPF broadcast the message that the some properties have changed, well, the ViewModel needs to raise the PropertyChanged event. So, I decided to go ahead and create a DynamicDecorator class, which would decorate my ViewModels with the “notify property changed” behavior.

Before assigning my ViewModel to the UI’s DataContext property, I’d use the decorator, somewhat like this:

DataContext = new DynamicDecorator(new InvoiceItemViewModel());

In reality, I had an interceptor in my repository that’d do the decoration part automatically for me, but that’s beyond the point here.

In other words, say I had the following code somewhere in my WPF app:

var item = GetItem();
item.ProductName = “Banana”;

The implementation of GetItem could look like this:

InvoiceItemViewModel GetItem()
{
return new InvoiceItemViewModel();
}

But then again, since WPF’s databinding is late-bound, GetItem, in my case, looked something like this:

The DynamicDtoDecorator simply intercepts access to the properties on the ViewModel it decorates, and it raises PropertyChanged when appropriate. I’ll show and explain the implementation of that class on my next post.

Wondering why the class is named Decorator, instead of Proxy? That’s also coming up in another post. Stay tuned!